1. Field of the Invention
Embodiments of the present invention relate to a multicylinder engine having a plurality of cylinders, and in particular to a multicylinder engine in which each cylinder is provided with an intake port with a throat having a narrowed portion that produces tumble flow in a combustion chamber.
2. Description of Related Art
Air-guided stratified charge combustion is known. For this type of combustion, tumble flow is produced in a combustion engine, and fuel is injected from an in-cylinder injector against the tumble flow, so that the penetration force of fuel spray in the combustion chamber is controlled, and a layer of an air-fuel mixture having a high fuel concentration is formed around an ignition plug. In air-guided stratified charge combustion, the tumble flow and the fuel spray are balanced with each other so as to realize appropriate stratification of the air-fuel mixture, and achieve favorable ignition performance and combustion quality. However, if the tumble flow and the fuel spray are not balanced with each other, intended ignition performance and combustion quality cannot be obtained. This is because, if the penetration force of the fuel spray is relatively weak with respect to the strength of the tumble flow, the air-fuel mixture does not sufficiently reach the ignition plug by the ignition timing. Conversely, if the penetration force of the fuel spray is relatively strong with respect to the strength of the tumble flow, the air-fuel mixture passes around the ignition plug. Further, in a multicylinder engine such as an internal combustion engine for an automobile, the degree of imbalance between the tumble flow and the fuel spray is likely to differ among the cylinders, which results in variations in combustion among the cylinders.
One reason why the degree of imbalance between the tumble flow and the fuel spray differs among the cylinders in a multicylinder engine is an error in shape of an intake port of each cylinder. A cylinder head in which intake ports are formed is generally produced by casting. The intake ports and the combustion chambers to which the intake ports are connected are formed by separate cores. The cores of each intake port and the combustion chamber to which they are connected are combined with each other, placed in a casting mold, and fixed by core prints. However, it is difficult to completely fix the cores during pouring, and a slight displacement or misalignment may arise between the core of the intake port and the core of the combustion chamber. When the intake port is a tumble production port that is shaped so as to produce tumble flow, the slight displacement largely influences the strength of the tumble flow produced in the combustion chamber. Since the intake ports of the respective cylinders are formed by separate cores, the degree of displacement of the intake port relative to the corresponding combustion chamber differs from one cylinder to another. Therefore, the strength of the tumble flow varies among the cylinders, and, consequently, the degree of imbalance between the tumble flow and fuel spray differs among the cylinders.
A technology to correct the imbalance between the tumble flow and the fuel spray for each cylinder is disclosed in Japanese Patent Application Publication No. 2002-276421 (JP 2002-276421 A). According to this technology, the fuel injection pressure of the in-cylinder injector is forced to be changed during stratified charge combustion operation, and resulting combustion fluctuations are detected. Then, the fuel injection timing is corrected for each cylinder to reduce the degree of combustion fluctuations. In a cylinder in which the penetration force of the fuel spray is relatively weak with respect to the strength of the tumble flow, the fuel injection timing is advanced, so that the air-fuel mixture can reach the vicinity of the ignition plug earlier, in time for the ignition timing. Conversely, in a cylinder in which the penetration force of the fuel spray is relatively strong with respect to the strength of the tumble flow, the fuel injection timing is retarded, so that the air-fuel mixture reaches the vicinity of the ignition plug with a delay in accordance with the ignition timing.